A new supported phase change material (PCM) made of paraffin impregnated by capillary forces in a compressed expansed natural graphite (CENG) matrix is presented. High loads of paraffin were obtained: from 65% to 95% weight depending upon the bulk graphite matrix density. Composite PCM/CENG thermal conductivities were found to be equivalent to those of the solo graphite matrix: from 4 to 70 W m(-1) K-1 instead of the 0.24 W m(-1) K-1 of the pure paraffin. Thermal Fewer and capacity of the composite are theoretically compared to those of conventional systems in the casi: of two usual external geometries: tubes and spherical hollow nodules. The CENG induced a decrease in overall solidification time and a stabilization of the thermal storage power. An optimization procedure of the composite composition was proposed according to the antagonistic behaviours of the thermal power and the thermal capacity with respect to the CENG content. Within tile usual external heat transfer coefficient range, the estimated CENG matrix optimized densities fell within the practicable range.